Lasers watch plant roots grow in transparent soil

The roots of plants are a pain to study because it is so hard to see what they are up to. Removing the soil damages them and can't be repeated to monitor growth. X-ray imaging does work, but it is slow and expensive. Last year botanists at the James Hutton Institute in Dundee, UK, partly cracked the problem when they developed transparent soil. Now they have teamed with imaging specialists at the University of Dundee to capture high-magnification 3D images of roots, with a laser.

The transparent soil combines transparent plastic particles soaked in a clear liquid. The plastic and the liquid have the same refractive index, so optically they behave as if they are a single material. This gives a good view of roots growing in the soil, but imaging their growth accurately requires adding fluorescent dyes that can affect plant metabolism, or genetically engineering plants to produce fluorescent proteins.

Building up slices

So Zhengyi Yang and colleagues at the university decided to use a laser to track the roots. They built a simple optical system that spread the beam from a blue-green laser into a thin sheet. The light passes straight through the transparent soil, but bounces off roots in all directions, like light scattered by tiny water droplets in fog. "By imaging perpendicular to the light sheet plane, we can get a 2D image with one shot," Yang says. Moving the sample up and down and capturing a series of 2D slices builds up a 3D image of the roots, like a series of scans builds up 3D images in a CT scanner.

The modest laser needed for the technique, known as light-sheet tomography, does not damage the roots. That let Yang's group take in-situ images in a sequence lasting up to 18 hours, after which the root tips they were monitoring grew out of range. The technique was good enough to observe root hairs and other important details, and can be used on most crop species. It's also relatively cheap. The system will let researchers monitor root growth and so develop crop strains that can absorb water and nutrients well under various conditions.

Robin Walker, an agronomist at Scotland's Rural College in Aberdeen, said the technique could be useful. "It allows you to look where the roots are growing and how that relates to nutrient uptake and water use," he says. "But you have to remember what's growing in a pot doesn't necessarily grow in a field."

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Roots laid bare (Image: James Hutton Institute and the University of Abertay Dundee)